In recent years, with development of information equipment, the needs for low-power and thin display devices have grown, so that extensive study and development have been made on display devices fitted to these needs. Of these display devices, a liquid crystal display devices has been developed actively as a display device capable of meeting the needs by electrically controlling alignment of liquid crystal molecules to change optical characteristic of the liquid crystal and has been brought into the commercial stage.
However, such a liquid crystal display device is accompanied with such problems that it has poor viewability of characters on a picture area due to a viewing angle or reflection light and that an eyestrain problem caused by flickering, low luminance, etc., of a light source is not sufficiently solved.
As a conventionally known display device which is low power consumption and thin and capable of solving the eyestrain problem, U.S. Pat. No. 3,612,758 has proposed an electrophoretic display device.
In the electrophoretic display device, a multiplicity of electrophoretic particles which are, e.g., positively charged and colored are dispersed in a space between a pair of substrates, each provided with an electrode, together with an electrophoretic dispersion liquid which is filled in the space and colored a color different from the color of the electrophoretic particles. Between the pair of substrates, a partition wall is formed so that it divides the space into a multiplicity of pixels along a planar direction of the substrates. By forming such a partition wall, it is possible to define the space between the pair of substrates while preventing movement of the electrophoretic particles in a pixel to other pixels and keeping uniform display.
In such an electrophoretic display device, when a positive-polarity voltage is applied to an observer's side electrode and a negative-polarity voltage is applied to an electrode on an opposite side, the positively charged electrophoretic particles are collected so as to cover the opposite side electrode, so that the color of the electrophoretic dispersion liquid (dispersion medium) is displayed when the electrophoretic display device is observed in such a state.
On the other hand, when a negative-polarity voltage is applied to the observer's side electrode and a positive-polarity voltage is applied to the opposite side electrode, the positively charged electrophoretic particles are collected so as to cover the observer's side electrode, so that the color of the electrophoretic particles is displayed when the electrophoretic display device is observed in such a state.
By performing such a drive of the electrophoretic display device on a pixel-by-pixel basis, any image or character is displayed by a multiplicity of pixels.
In a conventional electrophoretic display device, an electrophoretic dispersion liquid is prepared by adding, e.g., an electrifying agent or a dispersing agent into an insulating dispersion medium to impart a chargeability (electrification characteristic) and a dispersibility to electrophoretic particles. The chargeability of the electrophoretic particles results from creation of zeta potential at particle surface, and the dispersibility results from steric-exclusion effect by adsorption of the dispersing agent onto the particle surface.
U.S. Pat. No. 5,380,362 has disclosed such a method that a chargeability and a dispersibility are imparted to pigment particles in an electrophoretic dispersion liquid colored with a dye by using yellow pigment particles having an acidic site in combination with a charge control agent having a polymer chain containing a basic group.
Further, U.S. Pat. No. 6,590,696 has disclosed such a method that a chargeability and a dispersibility are imparted to particles in an electrophoretic dispersion liquid comprising a hydrocarbon solvent, particles having an acidic group (or a basic group), a basic (or acidic) polymer, and a compound having a nonionic polar group.
However, in the above described conventional electrophoretic display devices using electrophoretic dispersion liquids, additives have been precipitated at low temperatures in some cases. Particularly, in a low temperature environment of 0° C. to −20° C., when the electrophoretic display devices are driven, a polymeric additive is precipitated, thus causing a deterioration in display performance of the electrophoretic display devices.